A light emitting diode (LED) is a device in which a compound semiconductor such as GaAs, AlGaAs, GaN, and InGaInP is coated with a phosphor, and emits light having various colors. The LED converts a wavelength of light emitted from a light emitting diode (LED) to that of a required color using an outside phosphor.
Conventionally, a method which converts a wavelength of light of a blue LED to that of white light using a yellow color phosphor has been used. The white LED having such a structure has been widely used because it has a low cost and is structurally simple in principle.
However, there is a problem in which a color rendering index (CRI) is low because of lack of a spectrum of green and red due to the emission of a single yellow phosphor.
The CRI is an index which evaluates how well an object of unique color is seen as a suitable natural color in light of a light source. Such a CRI includes a general CRI Ra which is an average value averaged by eight colors of R1 to R8 set, and special CRIs R9 to R15. Such special CRIs include R9 as red, R10 as yellow, R12 as blue, and the like.
Recently, a technology in which the CRI is improved by mixing a red phosphor, a green phosphor, or the like as well as the yellow phosphor has been developed. However, although such a technology improves the general CRI Ra, there is a problem in that the value of R12 (blue) among the special CRIs is still low.